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For most any Ceph cluster, the user will want to--and may need to--change some Ceph configurations. These changes often may be warranted in order to alter performance to meet SLAs or to update default data resiliency settings.


Modify Ceph settings carefully, and review the Ceph configuration documentation before making any changes. Changing the settings could result in unhealthy daemons or even data loss if used incorrectly.

Required configurations

Rook and Ceph both strive to make configuration as easy as possible, but there are some configuration options which users are well advised to consider for any production cluster.

Default PG and PGP counts

The number of PGs and PGPs can be configured on a per-pool basis, but it is advised to set default values that are appropriate for your Ceph cluster. Appropriate values depend on the number of OSDs the user expects to have backing each pool. These can be configured by declaring pg_num and pgp_num parameters under CephBlockPool resource.

For determining the right value for pg_num please refer placement group sizing

In this example configuration, 128 PGs are applied to the pool:

kind: CephBlockPool
  name: ceph-block-pool-test
  namespace: rook-ceph
  deviceClass: hdd
    size: 3
    pg_num: '128' # create the pool with a pre-configured placement group number
    pgp_num: '128' # this should at least match `pg_num` so that all PGs are used

Ceph OSD and Pool config docs provide detailed information about how to tune these parameters.

Nautilus introduced the PG auto-scaler mgr module capable of automatically managing PG and PGP values for pools. Please see Ceph New in Nautilus: PG merging and autotuning for more information about this module.

The pg_autoscaler module is enabled by default.

To disable this module, in the CephCluster CR:

    - name: pg_autoscaler
      enabled: false

With that setting, the autoscaler will be enabled for all new pools. If you do not desire to have the autoscaler enabled for all new pools, you will need to use the Rook toolbox to enable the module and enable the autoscaling on individual pools.

Specifying configuration options

Toolbox + Ceph CLI

The most recommended way of configuring Ceph is to set Ceph's configuration directly. The first method for doing so is to use Ceph's CLI from the Rook toolbox pod. Using the toolbox pod is detailed here. From the toolbox, the user can change Ceph configurations, enable manager modules, create users and pools, and much more.

Ceph Dashboard

The Ceph Dashboard, examined in more detail here, is another way of setting some of Ceph's configuration directly. Configuration by the Ceph dashboard is recommended with the same priority as configuration via the Ceph CLI (above).

Advanced configuration via ceph.conf override ConfigMap

Setting configs via Ceph's CLI requires that at least one mon be available for the configs to be set, and setting configs via dashboard requires at least one mgr to be available. Ceph may also have a small number of very advanced settings that aren't able to be modified easily via CLI or dashboard. The least recommended method for configuring Ceph is intended as a last-resort fallback in situations like these. This is covered in detail here.